The invention herein described relates generally to speedometers for the sensing and display of boat speed and more particularly to a Global Positioning System (GPS) controlled marine speedometer unit which eliminates problems associated with fouling or jamming of impellers utilized to sense boat speed.
Marine speedometers, or knotmeters, heretofore have used water movement past an impeller or pilot tube to provide a signal related to the speed of a boat. One of the problems with impeller-type or pilot tube-type speedometers is that the impeller or pilot tube can become fouled both by barnacles, plants and debris in the water. Fouling of the impeller or pilot tube renders the speedometer useless and requires cleaning of the impeller or pilot tube to render the speedometer once again useful. In addition, impeller-type and pilot-tube type speedometers are inexact, particularly when a boat is skipping across the water such that flow of water across the impeller or pilot tube is intermittently interrupted.
The present invention provides a speedometer unit that uses a standard output of a position determining navigation unit such as a LORAN, GPS, SAT/NAV, OMEGA, or other type navigation unit. The speedometer includes a processor, such as a conventional microprocessor, that has an input for receiving speed information from the navigation unit, which speed information preferably is outputted according to the National Marine Electronics Association (NMEA) 0183 interface standard. The speed information, which according to such standard is outputted in knots per hour, may be converted to miles per hour for display on an analog or digital display disposed in a housing containing the processor.
In a preferred embodiment, the processor has a further input for connecting to an input device, such as a push button switch, for controlling an operational mode of the speedometer, for selecting the information displayed on the display and/or for resetting the speedometer. Preferably, the processor is programed to store the top speed encountered since the speedometer was reset. The pushbutton may be used in a speed display mode to switch between real time speed display and display of the top speed. The top speed memory may be reset, for example, by holding the pushbutton down for a minimum specified time greater than the depress time needed to switch between speed modes or other operational modes of the speedometer.
In a preferred embodiment, the speedometer is further provided with a tachometer, the function of which may be performed by the same processor used for processing speed, or by another processor. The tachometer receives an input signal from the boat""s engine which is related to the rotational speed of the engine. Preferably, the processor can be selectively set to control the number of active digits that are displayed on a display panel to provide for faster updates or for more precise readouts. For example, a four digit display may be set to provide four active digits at a slower update rate, two active digits at a faster update rate, or three active digits at an intermediate update rate. Provision is made to store in memory for recall the top RPM since the tachometer was last reset.
Further in accordance with the invention, the speedometer may have a further input for receiving an operating voltage of the boat""s engine and may operate in a voltage mode for display of the voltage on the display panel. Another operational mode may be provided to display boat heading based on the heading information received from the navigation unit. As above noted, the pushbutton may be used to switch between the several operational modes of the unit and/or to reset the unit.
The foregoing and other features of the invention are hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail one or more illustrative embodiments of the invention, such being indicative, however, of but one or a few of the various ways in which the principles of the invention may be employed.